1. Field of the Invention
The present invention relates to a method of making a sintered body for a rare earth magnet, and more particularly, the present invention relates to a method of making a sintered body for use in, for example, an R—Fe—B type magnet.
2. Description of the Related Art
A rare earth alloy sintered magnet (permanent magnet) is normally produced by compacting a powder of a rare earth alloy, sintering the resultant compact and then subjecting the sintered body to an aging treatment. To be a sintered magnet, the sintered body may be magnetized at an arbitrary time after having been subjected to the aging treatment. It should be noted that the “rare earth alloy sintered body” used herein means either a sintered body to be magnetized or a sintered body that has already been magnetized (i.e., a sintered magnet) according to the context.
Permanent magnets currently used extensively in various applications include a samarium-cobalt (Sm—Co) type magnet and a neodymium-iron-boron (Nd—Fe—B) type magnet. Among other things, an R—Fe—B type magnet (where R is at least one element selected from the rare earth elements including yttrium (Y) and is typically neodymium (Nd), Fe is iron and B is boron) is used more and more often in various types of electronic appliances. This is because an R—Fe—B type magnet exhibits a maximum energy product (BH)max that is higher than any of various other types of magnets and yet, the R—Fe—B type magnet is relatively inexpensive.
An R—Fe—B type sintered magnet includes a main phase consisting essentially of a tetragonal R2Fe14B compound, an R-rich phase including Nd, for example, and a B-rich phase. In an R—Fe—B type sintered magnet, a portion of Fe may be replaced with a transition metal element such as Co or Ni and a portion of B may be replaced with C. An R—Fe—B type sintered magnet, to which various preferred embodiments of the present invention are applicable, is described in U.S. Pat. Nos. 4,770,723 and 4,792,368, for example.
In the prior art, an R—Fe—B type alloy has been prepared as a material for such a magnet by an ingot casting process. In an ingot casting process, normally, rare earth metal, electrolytic iron and ferroboron alloy as respective start materials are melted by an induction heating process, and then the melt obtained in this manner is cooled relatively slowly in a casting mold, thereby preparing an alloy ingot.
Recently, a rapid quenching process such as a strip casting process or a centrifugal casting process has attracted much attention in the art. In a rapid quenching process, a molten alloy is brought into contact with, and relatively rapidly cooled and solidified by, the outer or inner surface of a single chill roller or a twin chill roller, a rotating chill disk or a rotating cylindrical casting mold, thereby making a rapidly solidified alloy, which is thinner than an alloy ingot, from the molten alloy. The rapidly solidified alloy prepared in this manner will be herein referred to as an “alloy flake”. The alloy flake produced by such a rapid quenching process normally has a thickness of about 0.03 mm to about 10 mm. According to the rapid quenching process, the molten alloy starts to be solidified from a surface thereof that has been in contact with the surface of the chill roller. That surface of the molten alloy will be herein referred to as a “roller contact surface”. Thus, in the rapid quenching process, columnar crystals grow in the thickness direction from the roller contact surface. As a result, the rapidly solidified alloy, made by a strip casting process or any other rapid quenching process, has a structure including an R2Fe14B crystalline phase and an R-rich phase. The R2Fe14B crystalline phase usually has a minor-axis size of about 0.1 μm to about 100 μm and a major-axis size of about 5 μm to about 500 μm. On the other hand, the R-rich phase, which is a non-magnetic phase including a rare earth element R at a relatively high concentration, is dispersed in the grain boundary between the R2Fe14B crystalline phases.
Compared to an alloy made by the conventional ingot casting process or die casting process (such an alloy will be herein referred to as an “ingot alloy”), the rapidly solidified alloy has been quenched and solidified in a shorter time (i.e., at a quench rate of about 102° C./sec to about 104° C./sec). Accordingly, the rapidly solidified alloy has a finer structure and a smaller average crystal grain size. In addition, in the rapidly solidified alloy, the grain boundary thereof has a greater area and the R-rich phase is dispersed broadly and thinly in the grain boundary. Thus, the rapidly solidified alloy also excels in the dispersiveness of the R-rich phase. Because the rapidly solidified alloy has the above-described advantageous features, a magnet with excellent magnetic properties can be made from the rapidly solidified alloy.
An alternative alloy preparation method called “Ca reduction process (or reduction/diffusion process)” is also known in the art. This process includes the processing and manufacturing steps of: adding metal calcium (Ca) and calcium chloride (CaCl) to either the mixture of at least one rare earth oxide, iron powder, pure boron powder and at least one of ferroboron powder and boron oxide at a predetermined ratio or a mixture including an alloy powder or mixed oxide of these constituent elements at a predetermined ratio; subjecting the resultant mixture to a reduction/diffusion treatment within an inert atmosphere; diluting the reactant obtained to make a slurry; and then treating the slurry with water. In this manner, a solid of an R—Fe—B type alloy can be obtained.
It should be noted that any small block of a solid alloy will be herein referred to as an “alloy block”. The “alloy block” may be any of various forms of solid alloys that include not only solidified alloys obtained by cooling a melt of a material alloy either slowly or rapidly (e.g., an alloy ingot prepared by the conventional ingot casting process or an alloy flake prepared by a quenching process such as a strip casting process) but also a solid alloy obtained by the Ca reduction process.
An alloy powder to be compacted is obtained by performing the processing steps of: coarsely pulverizing an alloy block in any of these forms by a hydrogen pulverization process, for example, and/or any of various mechanical milling processes (e.g., using a feather mill, power mill or disk mill); and finely pulverizing the resultant coarse powder (with a mean particle size of about 10 μm to about 500 μm) by a dry milling process using a jet mill, for example. The alloy powder to be compacted preferably has a mean particle size of about 1.5 μm to about 7 μm to achieve sufficient magnetic properties. It should be noted that the “mean particle size” of a powder herein refers to a mass median diameter (MMD) unless stated otherwise. The coarse powder may also be finely pulverized by using a ball mill or attritor.
A rare earth alloy powder is easily oxidizable, which is disadvantageous. A method of forming a thin oxide film on the surface of a rare earth alloy powder to avoid this problem was disclosed in Japanese Patent Gazette for Opposition No. 6-6728, which was originally filed by Sumitomo Special Metals Co., Ltd. on Jul. 24, 1986. According to another known method, the surface of a rare earth alloy powder may also be coated with a lubricant for that purpose. It should be noted that a rare earth alloy powder with no oxide film or lubricant coating thereon, a rare earth alloy powder covered with an oxide film and a rare earth alloy powder coated with a lubricant will all be referred to as a “rare earth alloy powder” collectively for the sake of simplicity. However, when the “composition of a rare earth alloy powder” is in question, the composition is that of the rare earth alloy powder itself, not the combination of the powder and the oxide film or lubricant coating.
Generally speaking, the material cost of the rare earth sintered magnet is relatively high. This is also true of an R—Fe—B type magnet including a lot of Fe as an inexpensive material. Thus, to cut down the material cost of the rare earth sintered magnet and not to waste valuable natural resources, methods of recycling defective rare earth alloy sintered bodies without remelting the sintered bodies have been researched and developed recently.
For example, Japanese Patent Publication No. 2746818 discloses a method of recycling a powder obtained by pulverizing the scrap of an Nd—Fe—B type alloy for a sintered magnet (which powder will be herein referred to as a “scrap powder”). In this method, the scrap powder of the Nd—Fe—B type alloy is mixed with a rare earth alloy powder (which is called “alloy B” in Japanese Patent Publication No. 2746818) to compensate for the oxidized portions of the material alloy and thereby improve the sinterability of the scrap powder.
Another method of recycling a scrap powder of an R—Fe—B type magnet is disclosed in Japanese Laid-Open Publication No. 11-329811. In that alternative method, an alloy powder, including an Nd2Fe14B phase as its main phase, is prepared by subjecting the scrap powder of the R—Fe—B type magnet to acid cleaning and Ca reduction processes, for example, and then mixed with a composition controlling alloy powder to improve the sinterability thereof.
According to these conventional recycling methods, however, an alloy powder, having a composition that is essentially different from that of the alloy powder as a material for the intended rare earth alloy sintered body, should be prepared. That is to say, since the “alloy B” powder or the composition controlling alloy powder needs to be prepared, the overall manufacturing process is adversely complicated. In addition, it is difficult to make a sintered body for a rare earth magnet from the alloy B powder or the composition controlling alloy powder alone. Also, even if a magnet could be made from such a powder, the magnetic properties of that magnet would be significantly inferior to the desired magnetic properties.